Isotope fractionation in different types of soil water

Abstract:

1. Background

Soil water ensures the transport of matter, e.g. nutrients, from the soil to the vegetation, and connected water bodies (groundwater, riparian areas, streamwater). Two distinct categories of soil water have been acknowledged: 1) mobile water, which moves downward through soil macropores eventually contributing to groundwater recharge and streamflow, and 2) bound water, which resides in the soil micropores and is mostly available to plants. Recent hydrological studies already consider these two different water-worlds in the understanding of hydrological processes. Nevertheless, the dynamics of the mixing processes that occur between the mobile and the bound waters during and after rain events is still poorly studied. Oxygen and hydrogen stable isotopes can be used to trace the origin of water, to determine its residence times within the soil compartment and to estimate the mixing of different waters in the soil. Nonetheless, the distribution of O and H isotopes throughout the soil matrix needs to be more clearly understood. For instance, as of today the influence of biogeochemical processes on the spatio-temporal variability of δ¹⁸O and δD of the soil solutions was rarely quantified. The oxygen and hydrogen exchanges between the soil water and the other soil compartments (living organisms, mineral, exchange capacity, organic matter) are still poorly studied and require deeper investigations. For instance, the weathering of silicate minerals produces O²⁺ in the soil solution, exchange capacity in acidic soils releases large quantities of H⁺into the soil solution and the degradation of the organic matter could also impact the oxygen and hydrogen isotope ratios of the soil water. But, are we able to quantify the contribution of these different processes to the oxygen and hydrogen isotopic composition of the soil water?

2. Aim and objectives

The aim of this study was to identify differences in the isotopic composition of 4 different types of soil water: drainage water, weakly bound water, moderately bound water and tightly bound water. Moreover, the study aims to determine relationships between soil properties and the oxygen and hydrogen isotope composition of those soil water types.

The 3 objectives of this study were to:

- test the suitability of two extraction methods, centrifugation and cryogenic vacuum distillation, commonly used for separating different soil water types and whether these methods alter the O and H isotopic composition of soil water. This objective will determine whether the following objectives can be addressed with confidence,

- determine whether different types of soil water, i.e.drainage, weakly bound, moderately bound and tightly bound water, can present distinct oxygen and hydrogen isotopic signatures,

- determine whether oxygen and hydrogen isotope signatures of the different types of water are related to soil bio-physico-chemical characteristics.

3. Methods

The O and H isotopic composition (δ¹⁸O and δD) of the different water types was determined for 10 soil samples that present contrasted pedological characteristics. Composite samples of A and B horizons were collected on 5 different forest sites. These sites are located in Luxembourg (sites W, R, H, E) and in Burgundy, north-eastern France (site B). W, H and E are situated in the experimental Attert River Basin (CAOS project). The forests growing on the sampling sites range from coniferous to mixed to deciduous tree stands. Among the soils are 3 Cambisols, 1 Luvisol and 1 Planosol. Moreover, the soils were chosen to cover a large range of soil organic matter content and soil textures: sand, sandy loam, loam and clay loam (Table 1).

For the experiment, the pre-treated soil samples (air-dried, sieved at 2 mm and homogenized) were filled into one litre, plastic bottles (5 sites x 2 horizons x 3 replicates + 2 controls = 32 bottles). The control consisted of pure silicate sand, which was sterilised before starting the experiment. One hundred litres of a reference tap water with a known O and H isotopic signature was collected and stored at 4°C to use in a series of saturation and drainage stages. The samples were initially left to saturate by capillarity with the reference tap water. After the saturation step, the mobile water in the samples was drained from the bottle. Next the samples were left in an incubator at 13°C for 33 days. The soil was left unsaturated during incubation to encourage aerobic microbial soil respiration (MSR). At the end of the incubation period the soils were re-saturated by capillarity and subsequently left to drain one final time. The initial and final drainage waters were collected, weighed, filtered (0.45 µm) and analysed for O and H isotope compositions. In addition, the loss of evaporation from the drainage water was estimated. The estimation was possible due to hourly weight measurements of tap water in a bottle that was left open to the atmosphere and placed next to the draining soils.

To separate weakly and moderately bound soil water, the samples were centrifuged at 20°C for 20 minutes, first using centrifuged parameters that correspond to a matric potential close to pF_2.5 then to pF_4.2. After the second centrifugation, the tightly bound water was extracted from the residual soil through static cryogenic vacuum distillation. The distillation was carried out in a vacuum of 10^-3mbar. A 65°C water bath evaporated water from the soil while the cold phase of pure liquid nitrogen (LN, -210°C) provoked condensation of the water into a collection tube. All water samples were filtered at 0.45 µm and analysed for their O and H isotope compositions. The results are expressed as the ratio of heavy to light isotopes of the extracted water compared to the Vienna Standard Mean Ocean Water (VSMOW) ratio.

The measurements of gravimetric water content (GWC) were carried out on subsamples of the soil after each extraction stage. In parallel to the water extractions for isotope analysis reference values of the GWC in the soils at pF_2.5 and pF_4.2were measured using ceramic plate extraction (CPE) and re-saturated air-dried soil. The gravimetric water contents obtained through centrifugation were then compared to these CPE reference values to determine whether centrifugation was effective.

Univariate ANOVA tests were carried out in Minitab 16 to test whether differences in the isotopic composition of water extracted from different soil types were statistically significant.

4. Results

4.1. Suitability of extraction methods

After the final drainage, all soils presented a moisture content high above pF_2.5. The ceramic plate extraction (CPE) confirmed that sandy soils with low total organic carbon (TOC) drained the most. Also, there was a linear relationship between the gravimetric water content (GWC) at pF_2.5 and the GWC at pF_4.2. When the soil was centrifuged instead of using CPE, the gravimetric water content at pF_2.5 and pF_4.2 also have a linear relationship. However, the GWC after centrifugation was higher than the GWC after CPE when the same pressure was applied, meaning that centrifugation was not as efficient as the CPE method. Important to note is the linear relationship between the soil moisture obtained through ceramic plate extraction and centrifugation, indicating a regular discrepancy between the two methods. Moreover, for many soils the water amount left in the soil after centrifugation to pF_4.2 was the same or even higher than the soil moisture after CPE to pF_2.5. This large difference in extraction efficiency indicates that centrifugation did in fact not extract any moderately bound water, i.e. any water between pF_2.5 and pF_4.2, for most soils. Only for the control sand did the two methods reach the same moisture level, which means that some soil characteristics could be responsible for the difference between methods. However, there were so far no correlations found for the moisture difference between the two extraction methods and soil characteristics, such as TOC, clay, silt and sand content. Soil structure changes provoked by handling of the bottles could have led to this discrepancy.

Cryogenic vacuum distillation removed varying percentages of the residual soil moisture depending on soil type. The extraction yield varied between 80 % and nearly 100 % with soil type, but the yield was not correlated with soil characteristics. Importantly, the cryogenic extraction yield and δD values of the tightly bound water were linearly correlated. This means that lower extraction yields lead to depletion of heavier isotopes in the extracted water compared to the reference water. The depletion was likely caused by incomplete evaporation of the soil water. During evaporation the heavier isotopes, such as deuterium, evaporate more slowly than light hydrogen isotopes - meaning that the water vapour becomes depleted in heavy isotopes while the liquid phase staying in the soil becomes enriched. Cryogenic vacuum distillation was assumed not to cause any fractionation. It is still unclear if the fractionation is only attributable to the cryogenic extraction yield. Soils with a yield close to 100 % still show fractionation of the extracted water compared to the reference water; it is possible that there are other factors that cause fractionation. One such factor could be the percentage of in situ water remaining in the soils after air-drying (0.2 to 7.9 %). This in situ water had a different isotopic composition from the reference water used for re-saturating the soils. It was however possible to determine that the fractionation was not due to the extraction of in situwater. It is also important to note that the cryogenically extracted water was in fact a mix of tightly bound and moderately bound water because centrifugation was not efficient.

4.2. O and H isotopic composition of the different soil water types

Overall there was small variation in δD values within the 3 water replicates from the same soil types extracted through centrifugation. As there was only 1 cryogenic distillation carried out per soil type, there is no indication of variance for the isotopic signature in cryogenically extracted water. The drainage water was significantly more enriched in deuterium than the reference tap water (p < 0.0005, Figure 1). This enrichment was largely attributed to evaporation during drainage. The isotopic signatures of the weakly bound water and the moderately bound water did not differ from the reference tap water (p = 0.44 and p = 0.118 respectively). The similarity in isotopic signature of the weakly and moderately bound waters to the reference tap water implied that these two bound waters were different from the drainage water. Furthermore, the isotopic signatures of these two bound waters were very similar. This similarity was likely due to the fact that centrifugation is not suitable to separate weakly from moderately bound water. The isotope composition of the water extracted with centrifugation and the tightly bound water differed noticeably; though it will be difficult to determine whether these differences were only caused by fractionation due to incomplete distillation or other factors.

4.3. Relationship between soil bio-physico-chemical properties and isotopic fractionation of soil waters

The control of the soil bio-physico-chemical properties on the isotopic fractionation of the different waters was large for drainage water while it was much less pronounced for the weakly and moderately bound waters. A full statistical analysis could not be carried out for tightly bound water as only 1 replicate per soil type was available. The drainage water showed significant differences in isotope compositions between horizons (p < 0.0005) and sites (p < 0.0005) as well as an interaction of the two factors (p = 0.003). In general soils with lower pH had a more deuterium rich drainage water. The difference in isotopic signatures between different pH levels is even larger in A horizons compared to B horizons. These differences between horizons were likely caused by the fact that A horizons usually drained less water. When little drainage water was collected, the percentage of evaporation from the total water was much higher, meaning that there was a higher potential for fractionation. Furthermore, A horizons and sandy soils were in general more acidic. It seems that this relationship caused a significant difference between soil types and an interaction between factors. The total organic carbon (TOC) content appears to be the main link between factors. A high TOC lowers the soil pH and traps water more efficiently, thus reducing the drainage capacity of the soil.

The moderately bound water did not present any significant differences in δD values when collected from different soil horizons (p = 0.078) nor from different sites (p = 0.24). Also, there did not appear to be any interaction between the two factors of horizon and site for δD values in moderately bound water (p = 0.633). Similarly, the weakly bound water, did not present any significant differences in δD values when collected from different horizons (p = 0.40), nor was there an interaction between factors of horizon and site (p = 0.109). However, the weakly bound water from the W site was significantly more depleted in deuterium than from the R (p = 0.035), H (p = 0.033) and E (p = 0.007) sites. Again, the TOC content seems to be the main difference among those soils. Therefore it is possible that microbial processes caused the differences in isotopic signatures. Nonetheless, these results need to be assessed with caution because centrifugation did not separate weakly and moderately bound water types efficiently.

5. Conclusion

The results found in this study suggest that different water types may have different hydrogen isotopic signatures and the isotopic signatures of a water type may vary between horizons and soil types. However, it is not yet possible to quantify the contribution of different bio-physico-chemical processes to the oxygen and hydrogen isotopic composition of the soil water because the available techniques for water separation are not reliable enough. Water extraction methods need to be improved before such quantifications can be attempted.

Acknowledgements

This study was part of the LORLUX project (Réseau transfrontalier de coopération de recherche Lorraine-Luxembourg sur la protection des ressources en Eau), co-funded by the Lorraine Region and the National Research Fund (FNR) of the Grand Duchy of Luxembourg and of the SOWAT project (Soil-water bypass and water connectivity at the headwater catchment scale) in the framework of the CORE research program (Contract no. C10/SR/799842). This study was carried out in cooperation with the French National Institute for Agricultural Research (INRA) - Nancy and the Swedish University of Agricultural Sciences (SLU), Uppsala. We would like to thank F. Barnich, J. Juilleret, C. Bréchet, C. Hossann, J. Frentress for their extensive field and laboratory work and for their helpful corrections of this abstract.